1. Technical Field of the Invention
This invention relates to solid waste disposal and more particularly to filtering devices for solid wastes.
2. Discussion of the Related Art
It is known that solid urban wastes must be subjected as a preliminary step to volumetrical separation and then to qualitative separation where they can be subjected with the necessary treatments to recycle recoverable portions and dispose of the non-recyclable portions.
Volumetric selection is usually done by means of rotating filters which comprise drums or cylinders having mantles provided with holes of various shapes and dimensions. The drum is rotated about its longitudinal axis and this rotation causes material placed inside to be separated according to the size of the material pieces.
In operation, the material pieces which are smaller than the dimensions of the holes pass therethrough, thereby discharging to the outside. On the other hand, the material pieces which exceed the hole dimensions continue across the free end of the drum for subsequent filtering or for final treatment or discharge.
In the case of solid urban wastes, the material placed in the rotating filter often contains scraps, fibers, plastic film, iron filings, and similar elongated bodies. These materials tend to wind themselves around the zone of the mantle of the cylinder between two contiguous holes, thereby clogging the separating holes.
This phenomenon of winding is caused by a filamentary or fibrous material landing across the drum segment between two adjacent holes and partly sticking out from them when these holes are at the lowest point during the rotation of the drum. During this rotation, these fibrous materials are carried in such a way as to move from the lowest to the highest zone. In this highest position, these materials tend to re-enter the drum interior due to the combined effect of the force of gravity ad the centrifugal force of the cylinder created by the aerodynamic effect in the direction opposite to that of the rotation of the drum. This re-entry occurs via both the hole from which they previously exited and from the hole adjacent beside the exited hole in the direction of rotation. This dual re-entry creates tangles around the two holes, thereby obstructing them.
This winding phenomenon is principally observed in the zones between the holes arranged longitudinally relative to one another. The transverse zones are less susceptible to winding, since in that radial direction the effect of the aerodynamic centrifugal force has little influence. Accordingly, the scraps and strings have the tendency to re-enter through the same holes they came out, thereby rendering the holes self-cleaning. Experience has shown that approximately 95% of the winding between two holes is observed in the longitudinal inter-spaces and 5% in the circumferential spaces.
Winding causes a rapid and progressive clogging of the holes with a consequent reduction of the efficiency of the filtering and the need for frequent and costly cleaning, with significant increase in cost.
In the Italian Patent No. 1,126,918, commonly assigned, winding is prevented by providing appendices in the form of cylindric or conical collars corresponding to all or part of the holes of the filtering surface and the outside of the cylinder. Although satisfactory, the solution envisioned by this patent shows significant inconveniences.
For example, considerable manufacturing costs are incurred, especially due to the fact that the collars are conical. Also, an appropriate projecting of the machines is needed on account of the machine weight, which in turn adds additional weight and therefore requires a greater power source. In spite of the cone shape of the collars, there is a tendency for a moderate quantity of material to re-enter the inside of the filtering cylinder in the phase of passing through some of the holes and the collars. In addition, the determination of the dimensions of the holes requires proceeding through successive approximations case-by-case, thereby frustrating any attempts to improve the filtering process. This is particularly true regarding filtering plants for solid urban waste. Finally, the perforated surface must be periodically changed after abrasion caused by the material, thereby increasing plant operational costs.
Accordingly, it is an object of the present invention to eliminate the phenomena of winding of scraps and/or strings around two contiguous holes of a filtering cylinder via a simple means which does not require special operations to clean the filter itself.
Other objects and advantages will become apparent from these drawings and specification which follow.